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"Exit" From Two 2di Duplex

Started by Grzrd, January 28, 2012, 11:22:23 AM

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Grzrd

If this question has been answered/discussed in a previous thread, apologies in advance ...

In Atlanta, the Downtown Connector ("DC") is an I-75/I-85 duplex.  The mileage markers on the DC reflect I-75 mileage, as do the exit numbers.  Yesterday, when driving northward toward the end of the duplex, the yellow "Exit" tabs on a DC BGS for "I-85 Greenville" caught my attention.  IOW I-85 "exits" from I-75 at that point (there is also a small exit sign at the split, either Exit 250 or 251 - I cannot remember exactly).

Is there any requirement to designate one 2di as exiting from another 2di at the end of a duplex?  Could GDOT just as easily installed an "I-85/Greenville" BGS without an exit tab and simply have not installed an exit sign at the split? Equality among 2dis?

In this instance, using the I-75 mileage for the DC makes sense.  IIRC I-85 is approximately 179 miles in Georgia; I-75's length in Georgia is much greater and the exits through Atlanta are generally in the 240s; motorist confusion avoided because no duplication of exit numbers on respective routes.

If this is simply a local practice by GDOT, I am curious as to how other states (maybe even state DOT districts?) handle this situation.


Alps

Well, I moved this thread before I saw there were two of them... Anyway, some states will and some states won't do it. You are supposed to designate one as the through route and have the other one exit, but to me there is one exception that should be added: when two routes start at MP 0 together. I'm thinking of I-20/59 entering Alabama when I say that, but there are many other cases. Otherwise, whichever route dominates the mileage and exit numbers, the other one should have the exit tab.

xonhulu

I'm sure the rationale is that the interchange needs to have some kind of exit number so it can be referred to by that number, and the sequence of exit numbers have to make some kind of sense for at least one of the routes.

I suppose they could give the exits on the duplex dual exit numbers, one reflecting I-75 mileage, one with I-85 mileage, but I think that would be needlessly confusing.  Better to just pick one interstate as the through route and continue its exit numbers, as GDOT is doing here.

NE2

Anything's better than how Tennessee does it (or used to do it):

(This is on I-40, approaching the I-24 overlap.)
pre-1945 Florida route log

I accept and respect your identity as long as it's not dumb shit like "identifying as a vaccinated attack helicopter".

vdeane

I'd love to tell you how NY does it, but we don't have any 2di multiplexes that would be considered normal.  I-90 and I-87 is the only 2di multiplex in NY right now, and both routes have screwed up exit numbers thanks to the Thruway.  Both have exit 1 following the Thruway exit; I-87 exits I-90, but this make sense as locals think of I-87 and the Northway as the same road (which they aren't, though they were planned to be).

In the case of I-86 and I-81, I-81 is the older route (naturally, as I-86 isn't designated on the multiplex yet; ironically, it can't, because I-81 isn't to interstate standards around NY 7).  The multiplex has I-81's numbers because I-81 superceeds NY 17 (both in status on paper and importance in reality).

The only other possible multiplex is I-86 and I-99 (not confirmed, though the I-86/US 15 interchange is set up for it).  In this case I would expect I-86 to have its numbers used, though it has seniority by miles as well (unlike I-86 and I-81).
Please note: All comments here represent my own personal opinion and do not reflect the official position of NYSDOT or its affiliates.

roadman65

What about I-29 in Kansas City?  Its "0" mile marker is not at its true southern terminus, but where it meets I-35 for its short duplex to I-70.  The road is actually 3 miles longer than the exits and mileposts. 

Most likely it is so those on I-35 do not get confused as they could sign the short duplex I-29's numbers, but being that I-35 southern terminus in Missouri is nearby you may get duplicate exit numbers.  However, some say that there is no official multiplex, but I think there is and the signs show it.  It would be better to truncate I-29 and have the same situation between I-75 and I-59 in Chatanooga, TN with "TO" shields along I-24 to make the connection.
Every day is a winding road, you just got to get used to it.

Sheryl Crowe

Alps

Quote from: roadman65 on January 28, 2012, 01:38:45 PM
What about I-29 in Kansas City?  Its "0" mile marker is not at its true southern terminus, but where it meets I-35 for its short duplex to I-70.  The road is actually 3 miles longer than the exits and mileposts. 

Most likely it is so those on I-35 do not get confused as they could sign the short duplex I-29's numbers, but being that I-35 southern terminus in Missouri is nearby you may get duplicate exit numbers.  However, some say that there is no official multiplex, but I think there is and the signs show it.  It would be better to truncate I-29 and have the same situation between I-75 and I-59 in Chatanooga, TN with "TO" shields along I-24 to make the connection.
I do think it's a multiplex, a very strange case though. We're waiting for it to become part of I-49 (:

roadman65

Quote from: Upside down frog in a triangle on January 28, 2012, 02:16:47 PM
Quote from: roadman65 on January 28, 2012, 01:38:45 PM
What about I-29 in Kansas City?  Its "0" mile marker is not at its true southern terminus, but where it meets I-35 for its short duplex to I-70.  The road is actually 3 miles longer than the exits and mileposts. 

Most likely it is so those on I-35 do not get confused as they could sign the short duplex I-29's numbers, but being that I-35 southern terminus in Missouri is nearby you may get duplicate exit numbers.  However, some say that there is no official multiplex, but I think there is and the signs show it.  It would be better to truncate I-29 and have the same situation between I-75 and I-59 in Chatanooga, TN with "TO" shields along I-24 to make the connection.
I do think it's a multiplex, a very strange case though. We're waiting for it to become part of I-49 (:

The State of Louisiana wants it badly.  It will happen as they want New Orleans and Canada to have a direct route.  Plus, I traveled the US 71 corridor and a freeway is needed.  It take almost a whole day to go from Kansas City to Shrevport with the winding curves and 55 mph of Arkansas.
Every day is a winding road, you just got to get used to it.

Sheryl Crowe

huskeroadgeek

I've often wondered about I-29 too-why if it is multiplexed with I-35 down to I-70(as the signage indicates)-why doesn't its mileage reflect it? I've always wondered if the extension along I-35 was added later, and they didn't want to change the mileage of it on the rest of the route. I also wonder if there is any other interstate in the country that is marked past its 0 mile marker.

US71

Quote from: Upside down frog in a triangle on January 28, 2012, 11:31:02 AM
Well, I moved this thread before I saw there were two of them... Anyway, some states will and some states won't do it. You are supposed to designate one as the through route and have the other one exit, but to me there is one exception that should be added: when two routes start at MP 0 together. I'm thinking of I-20/59 entering Alabama when I say that, but there are many other cases. Otherwise, whichever route dominates the mileage and exit numbers, the other one should have the exit tab.

I-70 and I-55 in Illinois have the same southern Terminus, but I-70 is counted as the exit when it splits at I-270.

At Effingham, I-57 is the "primary" route so all exits are numbered as I-57 exits. At the 57/70 split, 70 is signed as the exit.

At Bloomington, IL I-55 is the "primary" route, so I-74 is the exit.

At the risk of splitting the thread, in both cases the N-S is the "primary" route.  Is this always true or are there exceptions?
Like Alice I Try To Believe Three Impossible Things Before Breakfast

huskeroadgeek

Quote from: US71 on January 28, 2012, 02:31:05 PM
Quote from: Upside down frog in a triangle on January 28, 2012, 11:31:02 AM
Well, I moved this thread before I saw there were two of them... Anyway, some states will and some states won't do it. You are supposed to designate one as the through route and have the other one exit, but to me there is one exception that should be added: when two routes start at MP 0 together. I'm thinking of I-20/59 entering Alabama when I say that, but there are many other cases. Otherwise, whichever route dominates the mileage and exit numbers, the other one should have the exit tab.

I-70 and I-55 in Illinois have the same southern Terminus, but I-70 is counted as the exit when it splits at I-270.

At Effingham, I-57 is the "primary" route so all exits are numbered as I-57 exits. At the 57/70 split, 70 is signed as the exit.

At Bloomington, IL I-55 is the "primary" route, so I-74 is the exit.

At the risk of splitting the thread, in both cases the N-S is the "primary" route.  Is this always true or are there exceptions?
In Iowa, with the two multiplexes of I-80/I-29 and I-80/I-35, I-80 gets the exit numbers on both. That makes sense as I-80 is the longest interstate in the state. On the western junction with I-35 at Des Moines, the exit is numbered with I-80, even though I-80 is actually the route that is exiting at that point.

hbelkins

Kentucky has two concurrencies (I have come to detest the terms "duplex" and "multiplex") and they are I-75/I-71 and I-75/I-64.

In both cases I-75 has its mile markers continue as the "host" and the exits are where its "guests" depart.

Although there is an error on a couple of the new Clearview advance signs for the I-75 north/I-64 west split near Lexington. The Exit 118 tab is placed on the right side of the sign, over the I-75 lanes on the diagrammatic sign, instead of on 64's side. Older signage at the split and at the exit gore are correct.

In my travels, I have seen one instance where mileposts for both routes of a concurrency are used -- the US 13/US 50 bypass of Salisbury, Md.


Government would be tolerable if not for politicians and bureaucrats.

xonhulu

Quote from: roadman65 on January 28, 2012, 01:38:45 PM
What about I-29 in Kansas City?  Its "0" mile marker is not at its true southern terminus, but where it meets I-35 for its short duplex to I-70.  The road is actually 3 miles longer than the exits and mileposts. 

Most likely it is so those on I-35 do not get confused as they could sign the short duplex I-29's numbers, but being that I-35 southern terminus in Missouri is nearby you may get duplicate exit numbers.  However, some say that there is no official multiplex, but I think there is and the signs show it.  It would be better to truncate I-29 and have the same situation between I-75 and I-59 in Chatanooga, TN with "TO" shields along I-24 to make the connection.

What's the difference?  It's signed along with I-35 either way.  The inclusion of the word "TO" doesn't alter the navigational usefulness one bit.  In fact, it might be more reassuring for someone looking to get onto I-29 north to know they're already on it.  And only us roadgeeks care about the mileage markers.  I do agree, though, that I-29's "0" marker should be at it's actual beginning.  Again, though, kind of an irrelevant issue.

QuoteKentucky has two concurrencies (I have come to detest the terms "duplex" and "multiplex") and they are I-75/I-71 and I-75/I-64.

I agree, H.B.  I'd rather use "concurrency."  I think I've seen it more often in "official" usage, too.

If the Mt Hood Freeway had been built, then there would've been an Oregon concurrency (I-84/I-205) for me to compare to everyone's other examples, but alas, there are no Oregon concurrencies and I have nothing to offer!

NE2

Quote from: xonhulu on January 28, 2012, 02:52:49 PM
I agree, H.B.  I'd rather use "concurrency."  I think I've seen it more often in "official" usage, too.
Multiplex is almost never used officially (generally it's something written by a roadgeek who works there). I find overlap to be the most natural term, and it is used by a number of DOTs.
pre-1945 Florida route log

I accept and respect your identity as long as it's not dumb shit like "identifying as a vaccinated attack helicopter".

xonhulu

Quote from: NE2 on January 28, 2012, 03:02:44 PM
Multiplex is almost never used officially (generally it's something written by a roadgeek who works there). I find overlap to be the most natural term, and it is used by a number of DOTs.

"Overlap" is also better than "multiplex," but I dunno, somehow it doesn't seem like the most appropriate term to me.  "Concurrency" still works best for me.  However, if state DOT's prefer overlap, I'd rather use the term that's most official. 

Personally, I guess it doesn't make that much of a difference.

NE2

Quote from: xonhulu on January 28, 2012, 03:18:15 PM
"Overlap" is also better than "multiplex," but I dunno, somehow it doesn't seem like the most appropriate term to me.  "Concurrency" still works best for me.  However, if state DOT's prefer overlap, I'd rather use the term that's most official. 
Depends on the DOT. Some do use concurrency. Personally I find that a bit stilted, but that's just me.
pre-1945 Florida route log

I accept and respect your identity as long as it's not dumb shit like "identifying as a vaccinated attack helicopter".

roadman65

Quote from: NE2 on January 28, 2012, 03:36:22 PM
Quote from: xonhulu on January 28, 2012, 03:18:15 PM
"Overlap" is also better than "multiplex," but I dunno, somehow it doesn't seem like the most appropriate term to me.  "Concurrency" still works best for me.  However, if state DOT's prefer overlap, I'd rather use the term that's most official. 
Depends on the DOT. Some do use concurrency. Personally I find that a bit stilted, but that's just me.
Florida does use the term concurrency.
Every day is a winding road, you just got to get used to it.

Sheryl Crowe

NE2

Quote from: roadman65 on January 28, 2012, 03:58:45 PM
Florida does use the term concurrency.
Really? Show me one page or document from FDOT that uses it in the sense of two routes sharing mileage.
pre-1945 Florida route log

I accept and respect your identity as long as it's not dumb shit like "identifying as a vaccinated attack helicopter".

roadman65

I have to look, but back in the snail mail days, I think Fred Ferril was his name, used the term in his letters back to me about interesting findings regarding safety issues.  Anyway, he used it and so I believe Steve Homan (the FDOT 5 Public Info officer) emailed me with something to that effect a few years back.  My drive crashed since and lost all, but if I find the letter I will scan it and make it into a file and post it.
Every day is a winding road, you just got to get used to it.

Sheryl Crowe

NE2

Quote from: roadman65 on January 28, 2012, 04:33:51 PM
Anyway, he used it and so I believe Steve Homan (the FDOT 5 Public Info officer) emailed me with something to that effect a few years back.
Hey, I believe you that one or two workers used it. No need to scan it.
pre-1945 Florida route log

I accept and respect your identity as long as it's not dumb shit like "identifying as a vaccinated attack helicopter".

roadman65

#20
A concurrency, overlap, or coincidence in a road network is an instance of one physical road bearing two or more different highway, motorway, or other route numbers.[1] When it is two freeways that share the same right-of-way, it is sometimes called a common section or commons.[2][3]

Road enthusiasts often use the term multiplex–as well as the more specific duplex and triplex–to refer to such instances although those type are the more common instances.

Concurrency is a relatively common phenomenon: where two routes must pass through a single geological feature, or crowded city streets, it is often both economically and practically advantageous for them both to be accommodated on one road.

Often when two routes with exit numbers overlap (concurrency), one of the routes has its exit numbers dominate over the other and can sometimes result in having two exits of the same number, albeit far from each other along the same highway. An example of this is from the concurrency of I-94 and US 127 near Jackson, Michigan. The concurrent section of freeway has an exit with M-106, which is numbered Exit 139 using I-94's mileage-based numbers. US 127 also has another Exit 139 with the southern end of the US 127 business loop in Mount Pleasant, Michigan. However, there are also instances where the dominant exit number range is far more than the secondary route's highest exit number, for example the concurrency of I-75 and I-85 in Atlanta where I-75 is dominant–the exit numbers range from 242 to 251 while I-85's highest mile marker in Georgia is 179. I-40 in Tennessee (a neighboring state of Georgia), which is also concurrent with I-75, has the instance of its exit numbers in the concurrency range being higher than I-75's highest exit number in the state.
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This was taken from Wikipedia.
Every day is a winding road, you just got to get used to it.

Sheryl Crowe

TheStranger

California's only 2di concurrency (I-5 and I-10 along the southern tip of the Golden State Freeway - the portion that was never US 99), I'm not even sure if it has exit number signage up yet.

Not 2dis per se, but the Business 80/Route 99/US 50 concurrency through unsigned I-305 in Sacramento...contains NO exit number signage at all going east on US 50 through the junction!  Also, no exit numbering at all for the I-80/Business 80 & US 50 split in West Sacramento.  All the referenced signage was installed ca. 2008-2009, which makes this rather unusual.
Chris Sampang

InterstateNG

Quote from: US71 on January 28, 2012, 02:31:05 PM
At the risk of splitting the thread, in both cases the N-S is the "primary" route.  Is this always true or are there exceptions?

I checked 3 other concurrencies of this type:  the two along I-35 in OKC and 15/80 in SLC.  The N-S exit numbers were used in all three examples.
I demand an apology.

xonhulu

I was looking through a lot of OreDOT pages, and they seem to use "overlap."  However, it took forever to get back here as I started surfing a lot of other info on ODOT projects and got lost for awhile.  Funny how that happens.

PurdueBill

I-76/77 in Akron plays it both ways at the ends of the duplex.  Eastbound on 76, 77 SB leaves as a one-lane exit which is signed as Exit 23A throughout.  Westbound, 76 functions as the exit, with the two left lanes continuing as 77 NB, the #3 lane carrying 76 WB around the trumpet, and the #4 lane exit-only for East Ave, Exit 21A.  77 NB would be Exit 20, and is signed as such from 76 EB, but on the 76 WB/77 NB duplex as the signs begin for the breakup, 76 WB is signed with the EXIT ONLY signage despite the exits on the duplex having 76's exit numbers.  You can see the interesting signage on street view.  Exit 21A is 76's exit number, and then 76 itself is shown as an exit without a number, with 77 displayed as the thru route. (These signs were 2003 carbon-copy replacements of older signs that had the same peculiarities, including advertising TO I-277 before I-76 WB, which makes no sense for two reasons--1. the actual route should be shown before the "TO" one, and 2. 277 goes the opposite direction and not too many people are looking for it at this point.)




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